The G-Man (voiced by Michael Shapiro) is a mysterious recurring character in the Half-Life series of first-person shooter computer games. He is known to display peculiar behavior and capabilities beyond that of a normal human and his identity and motives remain almost completely unexplained. He plays the role of an overseer and employer, both observing the player as the games progress and pulling strings to control the outcome of specific events throughout the Half-Life saga. The G-Man's constant appearances in the Half-Life games, as well as his revealing monologues with series protagonist Gordon Freeman, imply he is of great importance and somewhat anchors the endeavors of the player. His mysterious nature has made him an icon of the Half-Life series.

Barney's name stemmed from the earlier alpha versions of Half-Life in which the model for the security guards held a resemblance to actor Don Knotts, inspiring comparisons with Knotts's character Barney Fife from The Andy Griffith Show, which in the United States has long been a disparaging term for an inept policeman or security guard. Initially, the "Barneys" were intended to be hostile NPCs who would attack the player.

In Half-Life: Blue Shift, the playable Barney progresses through Black Mesa to escape the events of the resonance cascade, and is able to do so, in contrast to Gordon Freeman and Adrian Shephard, who are held in stasis. In Half-Life 2, Barney works as a mole for the Lambda Resistance in the Combine Civil Protection Forces. He provides the player information in the first chapter, leading him to Kleiner and Vance, and in the end of the second chapter provides the player with his crowbar. The fact that Barney owes Gordon Freeman a beer is a running gag in the series.

Unlike the other Marines, he works with the increasingly distrustful scientists and guards in order to make it out of Black Mesa alive. Shephard and Freeman do cross paths during the game, but at that point Shephard is merely an observer to a climactic event from the original game, the time when Freeman jumps inside the Xen portal and disappears into Xen, however, if players tried to follow Freeman they would appear in a huge fall and subsequently die. He never fights alongside or against Gordon Freeman. The G-Man appears to take an interest in Shephard's movements, even before the Black Mesa Incident. As early as three months prior, Shephard spots the G-Man during the boot camp training mission. Shephard deactivates the thermonuclear warhead brought in by the Black Ops, but the G-Man reactivates it and Black Mesa is destroyed. In the end, the G-Man reveals that he has successfully argued for Shephard's life, detaining him in some unknown void. The G-Man expresses a degree of respect for Shephard, offering praise for his ability to "adapt and survive against all odds" which "rather reminds [the G-Man] of [himself]".

Dr. Rosenberg (voiced by Jon St. John) first appears in Half-Life: Decay. When Gina Cross and Colette Green first arrive at the test chamber's control room and are receiving instructions from Dr. Keller, Rosenberg interrupts and voices his concern to Keller over having the anti-mass spectrometer run above 90% capacity, which is past the safety buffer zone for the equipment. Dr. Keller, however, dismisses his concern and states that the administrator's orders for this were clear. He tells Rosenberg that he can either stay and watch the experiment or return to his labs by the train yards. Rosenberg remains, and shortly thereafter the resonance cascade occurs.

Immediately after the disaster, Rosenberg converses with Dr. Keller and makes clear that he believes their greatest responsibility should be to the safety of the people at Black Mesa. Although Keller thinks that they should attempt to reset the displacement fields first, he eventually agrees with Rosenberg and they come up with a plan to contact the military so that they can help and evacuate the facility as soon as possible. Gina and Colette escort Rosenberg through the Hazard Course to a satellite communications center on the surface, where he is able to transmit a distress signal. Dr. Rosenberg decides to wait there for the military, and this is the last time he is seen in Decay as Gina and Colette return below to assist Dr. Keller. However, his voice is heard once more in the game later on.

In Half-Life: Blue Shift, Rosenberg makes his first appearance during the Hazard Course tutorial, long before Calhoun encounters him in the train yards. He can be seen behind the observer's window during the duck-jump portion of the training.

Some time between Gina and Colette's last sight of Rosenberg in Decay and Calhoun's eventual rescue of the scientist in Blue Shift, he tries to enact an escape plan to get out of Black Mesa with the help of several other scientists. During this time, he is captured by soldiers and held captive in a freight car for questioning, while a colleague, Harold, is cornered and fatally wounded. Before Harold dies, Barney Calhoun discovers him, and he instructs Calhoun to find Dr. Rosenberg to help him with his plan. Calhoun is able to reach the train yards and free Dr. Rosenberg. Rosenberg informs him that their plan is to use the equipment in the prototype labs to teleport to safety.

He leads Calhoun to the unused part of the complex where two other scientists, Walter Bennett and Simmons, are already preparing the machine. Rosenberg instructs Calhoun that he must activate and align a relay device on Xen in order for them to be able to accurately set their destination. Calhoun travels to Xen and is successful in accomplishing this task, but after returning through the portal back to Earth (It is here that Gina and Colette in Decay, temporarily caught in a harmonic reflux, hear Rosenberg's voice calling Calhoun through the portal), they discover that they need another power cell to replenish the teleporter's power for their escape. Calhoun acquires a newly charged power cell from the lab's sub-basement and delivers it to Rosenberg and the others. Dr. Rosenberg then initiates the system and brings it online. They all narrowly avoid the military's invasion of the prototype labs, teleporting to the safety of an unnoticed access tunnel. They get into an SUV and leave Black Mesa.

Dr. Gina Cross is a Black Mesa scientist who first appears as the Holographic Assistant for Gordon Freeman in the Black Mesa's Hazard Course and then later as one half of the protagonist in Half-Life: Decay.

In Decay, Cross is the one who delivers the GG-3883 crystal sample to the delivery system and then heads to an area below the test chamber, where Colette Green is stationed, to fix a jam in the lift that allows the specimen to be delivered up to Gordon. After the Resonance Cascade occurs, Cross teams up with Dr. Green to battle their way through the now alien-infested facility. They first escort Rosenberg to the surface to contact the military, and then under the guidance of Richard Keller, they succeed in starting a resonance reversal to help lessen the effects of the dimensional rift. Gina Cross' fate, along with the rest of the survivors in Decay, remains unknown to the other Black Mesa survivors.

Dr. Colette Green is a Black Mesa scientist and one half of the protagonist team in Half-Life: Decay.

In Decay, Dr. Green's role in the experiment is to make preparations in a room below the test chamber and initiate the Anti-Mass Spectrometer to run at 105%. Dr. Gina Cross also enters the same room to fix a jam in the specimen delivery system's lift mechanism, meaning they are both in the same place when the Resonance Cascade finally occurs. Following the disaster, the two team up to fight their way through the facility for survival. They escort Dr. Rosenberg to the surface to call the military for help and then, with the help of Dr. Richard Keller, manage to start a resonance reversal to prevent the dimensional rift from becoming too large to be repaired.

The outcome for Dr. Green, along with the rest of the survivors in Decay, is unknown to the other Black Mesa survivors.

Dr. Richard Keller is a Black Mesa scientist, working with Colette and Gina. He appears in Half-Life: Decay.

Dr. Keller is a 55-year-old, senior scientist in a wheel chair. He gives missions to Colette and Gina during the game. Dr. Keller also condemns Gordon Freeman, and asks himself what Kleiner sees in him. His final fate is unknown.

Dr. Keller was originally going to be an antagonist, but the idea was scrapped.[citation needed]

Dr. Walter Bennet is a Black Mesa scientist. He is seen in Half-Life: Blue Shift and as a hologram in Half-Life: Opposing Force.

In Blue Shift, Walter is seen fixing the battery in Dr. Rosenberg's office, along with Simmons. They soon get it fixed with the help of Barney Calhoun, and start their teleportation out of Black Mesa. The four successfully make it out the facility, making Dr. Bennet one of the few known survivors of the incident. They open the gates and start their journey to the outside world with a SUV.

Dr. Bennet also appears in Opposing Force as a hologram. He introduces a Xen creature, barnacle, to Adrian.

Dr. Simmons is a Black Mesa scientist. He is seen in Half-Life: Blue Shift.

In Blue Shift, Simmons is seen fixing the battery in Dr. Rosenberg's office, along with Dr. Walter Bennet. They soon get it fixed with the help of Barney Calhoun, and start their teleportation out of Black Mesa. The four successfully make it out the facility, making Dr. Simmons one of the few known survivors of the incident. They open the gates and start their journey to the outside world with a SUV.

Simmons does not talk at all in the game, and his first name is unknown. Furthermore, his final fate is unknown like all of his colleagues.

Alyx Vance (voiced by Merle Dandridge) is a prominent figure in the human resistance against the rule of the alien race called the Combine and their human representative, Dr. Breen. She is a close friend and ally of Gordon Freeman. She is the daughter of Doctor Eli Vance and his deceased wife Azian.

Dr. Isaac Kleiner (voiced by Harry S. Robins), a Black Mesa survivor, is one of the leading scientists in the human resistance to the Combine. His character design is based on the generic "bald, glasses (Walter, as its model name suggests)" scientist model from the original Half-Life.

Dr. Kleiner was one of Gordon Freeman's professors at MIT, recommending him for employment at the Black Mesa to the Civilian Recruitment Division and working with him as part of the facility's Anomalous Materials team. He managed to survive the Resonance Cascade disaster of the first game with the aid of Eli Vance.

In Half-Life 2, he operates an underground lab in an abandoned Northern Petrol building. A teleportation system, developed jointly by Kleiner and Eli Vance, connects to Vance's facility, several miles away. As a pet, Dr. Kleiner keeps a debeaked headcrab he calls 'Lamarr' (after the 1930s actress and inventorHedy Lamarr).

In Episode One, Kleiner appears on the video screens previously reserved for Dr. Breen's propaganda and instructs survivors to evacuate City 17, also encouraging them to procreate. He rallies people to prepare for the Combine's retaliation, stating that several new technologies developed during their occupation would be deployed as soon as possible to help fight the Combine.

In Episode Two, Kleiner is working out of the White Forest Rocket Facility with Eli Vance and Arne Magnusson on a device intended to close the Combine superportal created by the Citadel's destruction. He mostly appears during radio transmissions while guiding Alyx and Gordon to White Forest, and argues bitterly with Magnusson, whom Vance states was Kleiner's rival for grant money at Black Mesa. Upon the discovery of the Borealis in Judith Mossman's decoded message, Kleiner expresses a wish to use the technology residing in the ship against the Combine, opposing Eli's vehement desire to destroy it in order to prevent "another Black Mesa".

Dr. Eli Vance (voiced by Robert Guillaume) is a physicist, researcher, and Harvard graduate, wears a prosthetic to replace his left leg beneath the knee, which was lost when he was attacked by a Bullsquid while helping Dr. Isaac Kleiner climb over a wall into a Combine city. This prosthetic makes reference to the "advanced knee replacement" Chell received in Portal. The only exception is he lost his leg and it is turned around. He is Alyx Vance's father; his late wife, Azian, died in the aftermath of the resonance cascade.

The leader of the Lambda Resistance, Dr. Vance was the first human being to make peaceful contact with the Vortigaunt species and thus the "first collaborator", quickly persuading the alien race to ally with humanity against the Combine invasion of Earth.

In Episode Two, Eli Vance works at the White Forest base before being killed by a Combine Advisor.

In Episode Two, Dr. Magnusson (voiced by John Aylward) runs the White Forest base and is described as a Black Mesa survivor. He gets on poorly with Dr. Kleiner due to their clashing personalities, as spelled out by their very names: 'Magnus' means 'great' in Latin, while 'klein' means 'small' in German and Dutch. Magnusson's peculiar personality seems to have gained him much respect from the Vortigaunts, such as his assistant Uriah, who makes awed references to him.

Magnusson also makes a remark to Freeman saying that if he successfully defends White Forest then he will forgive Freeman for an earlier incident in Black Mesa, involving his 'Microwave Casserole', a reference to a scene in the first Half-Life.

Dog is a hulking robot belonging to Alyx Vance, built by her father Eli to provide both companionship and to protect her. Alyx subsequently upgraded the robot into its current form. Despite its name, Dog is anthropomorphic in appearance. Dog provides support to Freeman during training with the Gravity Gun, and makes appearances several times after.

Dr. Judith Mossman (voiced by Michelle Forbes) first appears briefly in Red Letter Day, in which she is attempting to stabilize Gordon Freeman's damaged teleportation. In Black Mesa East, she meets Gordon Freeman at the entrance to the facility and tells him of the research that is being conducted there as well as her deep admiration for Freeman. Although she is obviously very fond of Dr. Eli Vance, she briefly quarrels with his daughter, Alyx Vance. Dr. Mossman disapproves of Alyx's light-hearted approach to science. It is clear that the two women dislike each other. Eli relates that Judith and Gordon Freeman applied for the same job at the Black Mesa Research Facility, but Freeman got it over her, due to his experience.

Alyx's dislike is seemingly justified when Judith betrays the resistance. Mossman had revealed the location of Black Mesa East to the Combine, allowing them to sweep the area and take Dr. Vance prisoner.

Later in the game, Alyx and Gordon capture Judith. They plan to rescue Dr. Vance via a Combine teleport, and Judith is the only one who can operate it. Dr. Mossman starts the machine, which requires several minutes to charge up. While Alyx and Gordon are distracted, Judith climbs into the teleport with Eli and they vanish. Moreover, Dr. Mossman has changed the teleport's destination coordinates — instead of Dr. Kleiner's lab, she has sent Dr. Vance and herself into the Citadel, the near-impregnable headquarters of the Combine. Alyx and Gordon barely manage to escape the overwhelming Combine forces by teleporting to Dr. Kleiner's location just before the teleporter explodes.

Later, in the chapter "Dark Energy", Dr. Vance rejects Dr. Breen's ultimatum. As a result, Dr. Breen tries to send Eli and Alyx Vance to a distant Combine off-world. Realizing that she has misplaced her trust in Dr. Breen, Dr. Mossman switches sides again. Threatening Dr. Breen with Alyx's EMP (the small electrical device used by Alyx to open doors or hack into computers) and telling him he is "all out of time" (a reference to the conversation she had with him when Breen says "So sorry, Judith. I'm all out of time."), she frees Dr. Vance as well as Alyx and Gordon. Judith and Alyx finally make peace, as Judith stays behind with Eli to guard him while Alyx and Freeman ascend an elevator to catch up with the fleeing Dr. Breen.

Dr. Mossman appears in Half-Life 2: Episode One through a video recording stored in the Citadel. She has survived the teleporter explosion along with Dr. Eli Vance and is now somewhere outside of City 17. In the recording, she is dressed in cold weather gear and is apparently sending back a report about a "project" she had been sent to investigate. The substance of the "Project" is not mentioned, but it is apparently not Combine in origin, as Mossman does not yet know if what she's looking for at the site will compromise certain work done by the Resistance "should the Combine discover its location." She is interrupted by an exploding Combine Door, which a Hunter and several Combine Elite soldiers pour through. Mossman successfully flees the scene before the Combine soldiers burst in, but her fate afterwards is currently unknown.

I'm fairly sure I've pinned down the location of the project. It's hard to say how much of it might have survived intact or if there is anything remaining that could compromise our work if it were discovered by the Combine. We'll need to take a close look at it, of course, but I should be able to give a better opinion within a few hours. If the site is where we think it is, it should be no more than...I'm gonna have to cut this short, we may have been spotted...

According to the conclusion of Episode Two, Judith Mossman's team found the Aperture Science research vessel Borealis buried in arctic ice after it "...Vanished, with all hands, and even part of the drydock!" as stated by Dr. Kleiner.

Colonel Odessa Cubbage (voiced by John Patrick Lowrie) is a member of the Resistance against the Combine who speaks in distinct Received Pronunciation. He wears a jacket with emblems on it indicating that he was possibly once a security officer as part of the University of Rochester Security Services. According to Raising the Bar, his model was based on the martial arts instructor for one of the game's developers, and the name was found in a spam filter.

Odessa Cubbage leads a small Resistance base and town, dubbed "New Little Odessa", in a coastal region outside City 17. Before arriving at New Little Odessa, the player can see Cubbage speaking with the G-Man by looking through a binocular spotting-scope device. When Gordon Freeman arrives at New Little Odessa en route to Nova Prospekt, Cubbage is briefing members on the use of the rocket launcher against Combine gunships. Cubbage entrusts the rocket launcher to Gordon and never turns up to fight himself, instead staying behind to attempt to contact another Resistance settlement.

Father Grigori (voiced by Jim French) is a Eastern Orthodox-like priest who appears throughout the Ravenholm chapter of Half-Life 2. He is the only human survivor encountered in Ravenholm.

He speaks enthusiastically about "tending to his flock", i.e. dispatching the remaining zombie inhabitants of the city with a hunting rifle and homemade traps while offering them consolatory words. He helps Gordon Freeman intermittently in Ravenholm, giving him a shotgun, combat tips and advice mingled with biblical quotations. Eventually, Grigori escorts Freeman through a cemetery infested with zombies to show him a hidden passage to the mines out of the haunted town. After waving Gordon off, Grigori continues fighting the hordes of enemies until he retreats into a nearby tomb, ignites a wall of fire around it and disappears, laughing maniacally.

Doctor Wallace Breen (voiced by Robert Culp) was the administrator of the Black Mesa Research Facility at the time of the "Black Mesa Incident," the events depicted in Half-Life, but he was neither seen nor mentioned by name. (He was instead referred to always as "the Administrator.") After the Seven Hour War, he "negotiated" a peace agreement with the Combine that saved humanity at the cost of enslavement. Doctor Breen was appointed as ruler of Earth — a puppet of the Combine, who have little physical presence on the planet.

The Half-Life 2 art book, Raising the Bar, has information that indicates Breen used, at least at one point of the planned story if not in the final version, a radio transmitter tower on the surface (i.e., not in Black Mesa) to communicate directly to the Combine and negotiate a surrender. Draft scripts for Half-Life 2 indicate that this would have been shown in an introductory segment to the game carried out through a series of projector slides. One of the slides would have shown Breen at the foot of a tower wearing a headset linked directly to it, with arms held wide and speaking to the skies.

Breen is alerted to the return of Gordon Freeman in Half-Life 2 when Gordon is temporarily teleported, by accident, to his office in the Citadel. Doctor Breen informs the Combine and immediately dispatches the forces at his disposal to capture Freeman and break the associated Resistance movement in City 17.

During Gordon Freeman's raid on the Citadel, Freeman is temporarily in the custody of Breen, until Judith Mossman turns against the administrator. During this period, Breen makes a very notable statement while in the presence of Alyx Vance and her father, Eli (who are also in his custody). He claims that Gordon "has proven a fine pawn to those who control him." He also comments that Gordon's services are "open to the highest bidder," and says he would understand if Gordon doesn't want to discuss it in front of his friends. These remarks imply that Breen may be aware of the mysterious G-Man and his influence over Freeman. It was also mentioned in one of the "Breencasts" to the Sector Seventeen Overwatch in Nova Prospekt; "I have good reason to believe that in the intervening years, he was in a state that precluded further development of covert skills."

When Judith Mossman frees Gordon Freeman and Alyx Vance in his office, Doctor Breen attacks Gordon by firing at him with the supercharged Gravity Gun; however, the charge doesn't kill him, and Breen leaves it behind while escaping. He then attempts to flee using a Combine teleporter. Gordon manages to stop him by destroying the Citadel's dark fusion reactor, which destroys the teleporter in a massive explosion, consuming Breen in the process.